1. Field of the Invention
The invention relates to systems for providing a flow of particles in a plasma processing chamber, and particularly to systems for providing a flow of reactive and/or energetic particles for processing a substrate. The method and system are preferably used during poly or metal etch.
2. Discussion of the Background Art
In many electrical device and solid state manufacturing processes, energetic-charged or energetic-neutral gas particles are used to process a substrate, such as a semiconductor wafer. In one implementation, the particles can be supplied by a plasma which is generated in a gas within a particle source powered by an inductive or a capacitive plasma coupling element. FIG. 1 illustrates an exemplary plasma processing system including an inductive plasma coupling element, i.e., helical RF coil 104. One known inductive plasma generating system is disclosed in U.S. Pat. No. 5,234,529, issued to Wayne L. Johnson, the inventor of the present application. The contents of that patent are incorporated herein by reference.
As illustrated in FIG. 1, gas is supplied to a process chamber 102 through gas inlets 112. An RF power source 110 having an output impedance RS supplies RF power to a plasma coupling element (e.g., RF coil 104) which, in turn, ionizes the gas, exciting it into a plasma state within a particular region (plasma generation region 108) of the process chamber 102. The RF power can, optionally, be coupled into the plasma coupling element through an impedance matching network (MN). The resulting plasma produces charged particles (i.e., ions and electrons) and neutral particles (neutral atoms or molecules). In some configurations, particles are accelerated by applying a voltage to a chuck on which a substrate (e.g., wafer 106) is mounted. The accelerated particles are emitted at the output 120 of the particle source 114. The particles are used to process, or facilitate the processing of, the substrate 106. For example, the particles can be used for ion-assisted deposition (IAD) or reactive ion-etching (RIE).
FIG. 2 schematically represents an example of a deposition system in which a film 304 is deposited on a substrate such as a wafer 106 by providing xe2x80x9cadd atomsxe2x80x9d 206 (i.e., atoms deposited, or xe2x80x9cadded,xe2x80x9d to form the film) from a film deposition source 202 (e.g., an evaporation source or a sputtering source). A flow of energetic particles 208 from a particle source 114 is directed toward the substrate 106 during deposition, and the particles 208 collide with the add atoms 206, thereby supplying energy to the add atoms. The energetic ions 208 increase the energy of the add atoms 206, thus add atoms 206 have higher mobility on the surface of the substrate 106. Accordingly, the add atoms 206 are more likely to settle and adhere to a region of low energy (e.g., a location in the film 304 where there is a void in a solid material). Increasing the energy of the add atoms 206 has the benefit of producing films of higher quality. For example, the films have higher density, larger grains, and fewer defects. Higher density films can be advantageous because they provide: (a) higher conductivity (in the case of conductive materials), and (b) conductivity which is more stable with respect to time. Dielectric films produced by this method have dielectric constants closer to solid density dielectric constants and have increased dielectric strength.
Although the energy of the add atoms can be increased by raising the substrate temperature, IAD reduces the need for raising the substrate temperature. In fact, ion bombardment, which adds an average of one electron volt (eV) of energy per add atom, has an effect comparable to increasing the substrate temperature by approximately 200xc2x0 C. The effectiveness of IAD can be increased further by supplying additional energy to the add atoms.
However, the addition of too much energy may have serious drawbacks on the processed substrates. If very high-energy atoms are used, the film can be damaged. For example, conventional ion sources for IAD provide ions having thousands of electron volts. Utilizing such high-energy ions has the disadvantage that it can cause implantation of the ions within the film. Implantation causes absorption of processing gases (e.g., argon used to provide the ions), which can deteriorate the films. Deterioration is caused (1) by reducing the density of the film, (2) by reducing the average grain size of the material in the film, or (3) by introducing additional defects into the film. Further, the addition of deeply implanted particles does not contribute to the desired mobility.
Reactive ion-etching can be used as part of a lithographic process for fabricating electronic circuits. An RIE process is described in Microchip Fabricationxe2x80x94A Practical Guide to Semiconductor Processing, by Peter Van Zant, and published by McGraw-Hill, Inc. The contents of the second and third editions of Van Zant are incorporated herein by reference. As designs become more compact across the horizontal plane of the substrate (i.e., in the x and y directions), and as designs increase in complexity, additional metalization layers are added (i.e., in the z direction). This change creates a need to process contact gaps and vias of varying heights and with higher aspect ratios. A description of the problems involved is discussed in the article entitled xe2x80x9cThe Interconnect Challenge: Filling Small, High Aspect Ratio Contact Holes,xe2x80x9d published in the August 1994 edition of Semiconductor International magazine. The contents of this article are incorporated herein by reference.
In known reactive ion etching systems, a film of material to be etched is first grown (e.g., by oxidation) or deposited (e.g., by sputtering or evaporation) on a substrate. A layer of resist is deposited on the film and developed into a desired pattern to form an etching mask, resulting in the structure shown in FIG. 3A. The substrate (e.g., a wafer) is then processed with ions 1602A generated by a plasma within a particle source in order to etch the film. Etching uncovers the substrate in the regions where etching is occurring, thereby leaving a patterned film on the substrate. The resist 308 is then removed (e.g., by using an oxide plasma), leaving a bare, patterned film. FIG. 3A depicts the processing of a substrate which includes a film 304 on a substrate body 302. The film 304 is etched by accelerated ions 1602A from a conventional particle source in order to form a narrow orifice or slot 1606. As the etching proceeds, electrons 1604 are attracted to, and adhere to, the resist layer 308, thereby defocusing the beam of ions 1602A and undesirably widening the orifice 1606 by side etching. In addition, the above-described side etching can cause narrowing of solid features such as conductive traces.
Since conventional sources of reactive particles produce ions with thousands of eV of energy, the etching selectivity of an etching process can be diminished (i.e., the process can undesirably etch the resist during etching of the grown or deposited film). In severe cases, high-energy particles can erode the resist enough to expose and undesirably etch portions of the film not intended for etching. Often, the resist is eroded near the edges of a feature, causing the feature to be reduced with respect to its intended size. Consequently, this can reduce the manufacturability of high-resolution patterns containing small features. Furthermore, high-energy particles can become implanted in the substrate body, or in a lower film beneath the film being etched, thereby degrading the electrical properties of the substrate body or the lower film.
Although the present invention is preferably applied to poly and metal etch processes, analogous problems exist when etching high aspect ratio holes in dielectric materials such as silicon dioxide. The process of etching high aspect ratio holes is further discussed in U.S. Pat. No. 5,468,339 to Gupta et al., entitled xe2x80x9cPlasma Etch Processxe2x80x9d in which 10:1 aspect ratios are described; in U.S. Pat. No. 5,468,340 to Gupta et al., entitled xe2x80x9cHighly Selective High Aspect Ratio Oxide Etch Method and Products Made by the Processxe2x80x9d; in U.S. Pat. No. 5,428,243 to Wylie, entitled xe2x80x9cBipolar Transistor with a Self-Aligned Heavily Doped Collector Region and Base Link Regionsxe2x80x9d in which holes of different depth are etched simultaneously; and in U.S. Pat. No. 4,717,448 to Cox et al., entitled xe2x80x9cReactive Ion Etch Chemistry for Providing Deep Vertical Trenches in Semiconductor Substrates.xe2x80x9d The contents of these patents are incorporated herein by reference.
A relatively recent source of extensive information on ion sources is the special issue of the Review of Scientific Instruments, Vol. 61, No. 1, Part II, January 1990. Moreover, Kaufman (U.S. Pat. No. 3,156,090) describes a two-grid ion source intended for use as an ion rocket engine and includes a neutralizer filament. Ikeda et al. (U.S. Pat. No. 4,243,1981) describe a parallel planar etching apparatus in which a first electrode includes a metallic mesh and a second electrode is a metallic plate. The target is placed outside the mesh electrode. Some of the plasma xe2x80x9cleaks down through the meshes of the mesh electrodexe2x80x9d and contacts the surface of a target. Harper and Kaufman (U.S. Pat. No. 4,259,145) describe a single extraction grid to form an ion beam with ion energies in the range from 10 to 100 eV. Cuomo and Harper (U.S. Pat. No. 4,351,712) describe an ion source, similar to the source described by Harper and Kaufman, with an ion energy source range between 30 and 180 eV. Ono and Matsuo (U.S. Pat. No. 4,450,032) describe an ion source resembling a shower head. The accelerating voltage is in the range from 200 to 1000 volts. Matsuo and Ono (U.S. Pat. No. 4,492,620) describe an adaption of the apparatus described in U.S. Pat. No. 4,450,031 which is modified to include an acceleration voltage as low as 100 V. Keller and Coultas (U.S. Pat. No. 5,206,516) use a three-electrode electrode decelerator lens to obtain a 25 eV ion beam from a much more energetic beam.
xe2x80x9cA Laboratory Simulation of the Ionospheric Plasmaxe2x80x9d by Pigache, AIAA Journ., 11 (2), pp. 129-30 (1973) describes a plasma wind tunnel that includes a single-grid 20 eV ion source. xe2x80x9cIon Beam Divergence Characteristics of Two-Grid Accelerator Systemsxe2x80x9d by Aston et al., AIAA Journ. 16 (5), pp. 516-24, (1978) report on the xe2x80x9cfirst comprehensive investigation of two-grid accelerator systems.xe2x80x9d xe2x80x9cIon Milling for Semiconductor Production Processesxe2x80x9d by Bollinger, Solid State Technology, pp. 66-70, (1977) discusses ion milling within the context of semiconductor processing in an early general article. xe2x80x9cLow Energy Ion Beam Etchingxe2x80x9d by Harper et al., J Electrochem Soc: Solid-State, 128 (5), pp. 1077-83, (1981) describe a single-grid ion source with beam energy less than 100 eV. xe2x80x9cIon Sources for Ion Machining Operationsxe2x80x9d by Kaufman et al., AIAA Journ., 15 (6), pp. 843-47, (1977) treat ion sources for industrial milling. xe2x80x9cTechnology of Ion Beam Sources Used in Sputteringxe2x80x9d by Kaufman, J. Vac. Sci. Technology, 15 (2), pp. 272-6, (1978) treats the use of ion beams for sputtering. xe2x80x9cBroad-Beam Ion Sourcesxe2x80x9d by Kaufman, Rev. Sci. Instrum., 61 (1), pp. 230-5, (1990) discusses the technology of broad-beam ion sources. Both gridded and grid-less ion sources are considered.
Tsuchimoto (U.S. Pat. No. 4,123,316) discloses a plasma processor in which plasma diffuses from a plasma generating chamber through a plasma output port into a plasma processing chamber. The distance from the plasma output port to a target wafer is to be less than the mean free path of the gas remaining in the processing chamber. For deposition procedures, the potential differences between the plasma within the plasma generating chamber and a target wafer may be only a few volts. xe2x80x9cPlasma Stream Transport Method . . . xe2x80x9d by Tsuchimoto, J. Vac. Sci. Technology, 15 (1), pp. 70-3 (1978) describes a plasma source from which the plasma effuses with thermal velocity to a target. The plasma stream is confined by a coaxial magnetic field. A later article by Tsuchimoto entitled xe2x80x9cPlasma Stream Transport Method . . . xe2x80x9d, 15 (5), pp. 1730-3, (1978) considers neutralization of the plasma stream described in his earlier (15 (1)) article. xe2x80x9cOperation Modes and Its Optical Measurements of Plasma Stream Transport . . . xe2x80x9d by Tsuchimoto, 17 (6), pp. 1336-40, (1980) presents additional details. Cuomo and Kaufman (U.S. Pat. No. 4,451,890) describe a Hall ion generator that produces a self-neutralizing low energy, high intensity ion beam. The anode-cathode potential difference is in the range from 30 to 50 volts.
Lee et al. (U.S. Pat. No. 4,652,795) disclose passing a plasma out of the plasma generating chamber through a large aperture. Anode and cathode voltages are independently variable. Sekiguchi and Mito (U.S. Pat. No. 4,664,747) describe an RF or microwave plasma generator in which radiation and/or active species from the plasma are conducted to a target positioned outside the plasma discharge space. A mesh extraction electrode may be used, but configurations with no extraction electrode are also considered. Zarowin and Bollinger (U.S. Pat. No. 5,290,382) describe an RF or microwave excited inductively coupled plasma generator with an outlet port through which the plasma flows downstream from the plasma chamber to a substrate. An xe2x80x9cinteractive flangexe2x80x9d provides a surface separate from the substrate to consume the active species and thereby controls the effective beams cross-section.
Chen et al. (U.S. Pat. No. 5,469,955) disclose a multi-chambered apparatus with which a neutral beam may be produced. A plasma is produced in an RF plasma generation chamber and, downstream, the plasma diffuses through a large aperture into a quiescent plasma chamber. A pseudo-neutral beam is produced in a third chamber. A particle energy xe2x80x9cbelow that which causes crystal lattice damage in semiconductor materialsxe2x80x9d is possible. xe2x80x9cFormation of MOS Gates by Rapid Thermal/Microwave Remote Plasma Processes . . . xe2x80x9d by Moslehi, IEEE Electron Device Letters, EDL 8 (9), pp. 421-4, (1987) describes a system in which a plasma is generated in a microwave cavity. The remote microwave plasma allows selective, controlled generation of specific plasma species.
Rogoff (U.S. Pat. No. 4,090,856) separates isotopes by first selectively ionizing one isotope in a partially bounded region. Ambipolar diffusion of the charged isotope toward the enclosure walls occurs. The gas adjacent to the boundary, enriched in the desired isotope, is substantially neutralized at the walls and is then separated from the remaining mixture. Chen (U.S. Pat. No. 4,297,191), and Bridges (U.S. Pat. Nos. 4,545,878 and 4,563,258) are similar.
Accordingly, there is a need for an apparatus and method which can provide lowenergy ions and neutral particles. In particular, it is necessary to provide a flow of particles with energies significantly less than 1,000 eV, in order to process a substrate without damaging either the substrate or films on the substrate.
It is therefore an object of the invention to provide a system and method which can provide low-energy particle beams for processing a substrate.
In accordance with one aspect of the invention, a plasma with a density gradient is utilized to accelerate ions. In particular, a particle source according to the invention can be configured to produce ions having a desired energy and, therefore, a desired speed and/or selectivity. If the chosen ion speed is low enough and the distance between a substrate being processed and the particle source is large enough, the ions can have sufficient time to recombine with free electrons (i.e., electrons which are not bound to atoms) before striking the substrate, thereby producing accelerated (i.e., energetic) neutral particles. This combination process occurs in a charge exchange region of the processing system.
According to an advantageous aspect of the invention, a plasma density gradient is produced by applying a non-uniform magnetic field to the plasma. Since the density nP of a plasma is dependent upon the magnetic field applied to it, a magnetic field with a gradient can be used to provide a plasma with a density gradient. The plasma density gradient produces an electric field, thereby creating a voltage between (1) where ions are created and (2) where they exit the particle source. This voltage, in turn, accelerates the ions. The energy of the ions is dependent upon the voltage induced by the electric field produced by the plasma density gradient. The energy is later passed in a charge exchange process when a charged ion hits a neutral ion.
According to another advantageous aspect of the invention, non-uniform RF power is applied to the plasma, thereby producing a non-uniform plasma density np which, in turn, produces an electric field (and a voltage associated with the electric field) in order to accelerate ions.
It is yet another object of the present invention to reduce the coupling effect between the RF source and the ions in a plasma. By reducing the coupling through RF shielding, ions are more easily directed down toward a processing area. This enables a greater wafer processing uniformity. Likewise, by providing a density gradient, it is also possible to produce particle acceleration without using a biased chuck. This change simplifies the system by reducing the amount of cooperative control that is needed between a first RF source applied to the RF coil and a second RF source applied to the chuck. This simplification reduces a cost of the overall system.
The invention uses a plasma density gradient to provide a flow of particles that can be used to process a substrate. The system utilizes a high density (i.e., a large number of particles per unit volume) stream of low-energy particles to process the substrate as effectively as a low density stream of high-energy particles. Moreover, the high density stream causes less damage than the low-density stream of high-energy particles. Thus, the present invention overcomes deficiencies in conventional systems and provides significant benefits, e.g., by improving the quality of deposited films. More specifically, (1) improved film morphology (e.g., less hillock formation) can be obtained, (2) higher and more stable values of conductivity can be obtained in conducting layers, and (3) electromigration effects can be reduced. Furthermore, substrate damage during etching of films can be reduced, resulting in fewer device defects. As a result, manufacturing yield is increased and manufacturing costs are reduced.